ESTRO 2023 - Abstract Book

S507

Sunday 14 May 2023

ESTRO 2023

leading to 10 error scenarios. The original and error plans were imported into RayStation v11B for dose calculation using reference beam data. For all plans, the near-max spinal cord dose (D0.03cc,SC) and dose to 90% of PTV (D90,PTV) were calculated. To assess impact of the individual errors, the changes in D0.03cc,SC and D90,PTV in the error plans versus in the unedited plan were evaluated. Dose difference maps between dose distributions of all error plans and the unedited plan were calculated for all 65 centres. An average of the difference maps ( Δ Dmap) for all centres was calculated to visually highlight regions consistently impacted by the error. Results Histograms showing increased D0.03cc,SC due to MLC retraction errors, and decreased in D90,PTV due to MLC contraction are shown for all centres (Fig 1). The dosimetric impact and its spread increase with error , with as low as 0.3mm retraction error causing >5% increase in D0.03cc,SC for some centres. Difference in the mean of each HDMLC and MMLC histogram pair was <0.5%. The Δ Dmap for 0.5mm retraction and contraction errors respectively show the impact is mainly clustered around the boundary between critical OARs and PTV (Fig 2).